1. Field of the Invention
The present invention relates to lenses which are suitable for use as a finite conjugate projection lens system, and, more particularly, to a small-size, high-performance, wide-angle reader-printer lens for use in a facsimile machine, an image scanner, a microfilm machine, etc.
In order to attain a high resolution of the order of 7 to 10 .mu.m on the image side, lenses used in facsimile machines, image scanners, microfilm machines, etc., are required to provide a high contrast ratio at high spatial frequencies. In addition, such lenses must have very little or no vignetting in order to minimize the decrease in the quantity of marginal light. It is also necessary to compensate various aberrations to a satisfactorily small level.
In order to meet all of these requirements, Gaussian-type lenses have been traditionally employed for these applications. Examples of such lenses are disclosed in U.S. Pat. Nos. 4,784,480 and 4,426,137. A typical Gaussian-type lens system is composed of two groups wherein one group is located preceding the stop and the other group is located behind the stop. Each group has two sub-groups (inner and outer) and the entire lens usually contains 6 to 8 lens elements. Typical Gaussian-type lenses have two doublets or a doublet and a singlet in each group and are symmetrical with respect to the sequence of the doublets and singlets about an aperture stop and also in terms of the signs of the powers of the lens elements. For example, this lens type often has a positive lens element as a first or outer sub-group of the front group, and a cemented doublet as a second or inner sub-group of the first group. Because the most typical type of Gaussian-type lenses is generally symmetrical about an aperture stop, the first or inner sub-group of the rear group is usually also a cemented doublet. The second or outer sub-group of the rear group typically comprises one positive lens element. Sometimes the second doublet is uncemented to improve the lens performance, but this results in greater sensitivity to tolerance errors. Sometimes more than one lens element follows the second doublet. For example, the last positive lens element may be split into two or more single lens elements, or may be converted into a cemented doublet, resulting in a better performance at the expense of having more lens elements. Examples of such lenses are described in U.S. Pat. Nos. 4,773,746 and 3,815,974.
The typical Gaussian-type lens system has a large amount of comatic flare at middle-angle positions and very large amount of oblique spherical aberration at the maximum angles. In addition, a major problem with using the conventional Gaussian type lens such as disclosed in U.S. Pat. No. 4,784,480 in a reader printer is that small changes in the element thicknesses of the first doublet produces relatively large changes in image distance and field curvature. Because magnification as well as object-to-image distance are fixed with little or no adjustment tolerance in most microfilm reader printers, this image distance sensitivity causes the first doublet of a conventional design to have either very tight tolerances on the thicknesses of both elements or the thicknesses of both elements comprising the first doublet must be "matched" so that the total thickness of the doublet is held to tight tolerances. Also, typical Gaussian-type lenses tend to suffer from a large amount of inward field curvature, causing additional image quality deterioration.
U.S. Pat. No. 4,671,627 discloses a two-group lens configuration of the same general type described above that has an overall symmetrical shape with respect to a diaphragm but which contains 8 lens elements.
U.S. Pat. No. 2,171,640 discloses an atypical variation of the Gaussian type lens which can be characterized in terms of its elements as (+-)- stop (-+)+ configuration. For analysis purposes, the disclosed embodiment of this lens was scaled to have the same magnification, object-to-image distance, coverage and an F-number as one of the illustrative embodiments of the present invention. The analysis showed that this prior art lens has an unacceptable amount of astigmatism at the field coverage achievable by lenses according to our invention. In addition, this prior art lens suffered from much larger amount of vignetting than that of the present invention.